Tools > Sync is the menu item for the Sync extension on Windows versions of Firefox. You can also access the Sync controls through Tools > Options > Sync

Setting up Sync on additional devices is slightly different because you don't need to setup a Sync Account as you did initially.
https://support.mozilla.com/en-US/kb/How+to+sync+Firefox+settings+between+computers#Step_2_Set_up_Firefox_Sync_on_other_installations_of_Firefox_

When you select Set Up Firefox Sync on the 2nd and subsequent devices, you need to select - I'm already using Sync on another computer.

The Secret Phrase shown in the screenshots on that page was renamed to Sync Key, that is what you'll see in Firefox Sync 1.5+ versions of the extension. Write that "key"down somewhere outside of Firefox so you don't lose it, in case something corrupts your computer and you can't open Firefox. That is what Firefox uses to encrypt and decrypt your personal data going to and from the Sync server. No one except you can read your personal data, and without that "key" you won't be able to read it either. You can reset your Sync Key of you need to, but when you do that all your data is wiped from the Sync server and allow you to start all over without creating a new account.

Thanks for your attention to this.

I believe I set it Firefox Sync correctly on both computers initially, and both computers are picking up Sync information from each other, but there is inconsistency between the computers.

Some Bookmark folders and Bookmarks are coming through on the PC, some aren't.

Eg. there is a folder on in my Mac FF which came over, but does not contain any bookmarks.

I'm not discerning a pattern yet as to what is happening.

Firefox Sync doesn't synchronize everything all at once, it does it in short bursts, so it might take awhile to get everything synchronized completely the first time. I have noticed a randomness in the manner in which the Sync process works with bookmarks and bookmark folders, but it eventually sorts itself out over a period of time. It is worse when you are starting from scratch with a new account and with multiple computers, than it is when you Sync one computer for awhile and then setup Sync on a 2nd or 3rd device.

Another factor affecting how quickly new installations of Sync complete the initial "sync" is the Sync server load, holiday weekends like this probably cause a bigger load of users on the server, thus slowing down the process for new users who need to upload & download larger amounts of data than existing users are sync'ing. I wouldn't be surprised if the process worked a lot faster well after midnight PST, when fewer Firefox users are online (unless they're like me who doesn't go to bed until the sun comes up in the morning, LOL).

My suggestion is to leave Firefox running on both computers for a few hours, with Sync "connected" and see if those "missing" bookmarks and folders appear later this evening. A large collection of bookmarks takes a lot longer than a small collection does, to synchronize. I have 3 separate bookmarks collections, in separate Firefox Profiles. The pRon bookmarks don't get sync'd at all, I do that type of surfing on only one PC; whereas, my personal and professional bookmarks are in separate Profiles, too, and those are sync'd with two different Sync accounts and to my Netbook. That cuts down on the size of any bookmarks file and speeds the sync process; and also keeps passwords and browsing history separate.

Thanks for the thorough and insightful answer, I'll be patient and see how it plays out over the next week or so.

The browsing History seems to be syncing instantly, a bookmark I just added to my Bookmarks Toolbar synced instantly, so for the most part, things are working as I'd expect.